High speed positive displacement motor

ABSTRACT

According to the present invention, a downhole motor converts hydraulic energy from drilling fluid passing through the motor into mechanical power that is useful to rotate a drillbit. The downhole motor includes a tubular housing and a stator mounted within the tubular housing. A rotor is positioned within the stator for rotation relative thereto. The stator has a central first axis and the rotor has a central second axis. The rotor is rotated about the second axis and nutated about the first axis within the stator by drilling fluid passing through the stator. The rotor further has a concentric lower end portion thereon. A cylindrical coupling is disposed within the tubular housing beneath the rotor and has an axis substantially aligned with the first axis and an inner cylindrical opening the axis of which is substantially aligned with the second axis, the opening being adapted for receiving the lower end portion of the rotor. A first bearing assembly is disposed in the opening of the coupling for rotatably receiving and supporting the lower end portion of the rotor, whereby nutation of the lower end portion of the rotor about the first axis induces rotation of the coupling about the first axis. A second bearing assembly is disposed between the coupling and the tubular housing for supporting rotation of the coupling relative to the tubular housing. A transmission shaft and bit box are connected to the coupling for rotation. In this manner, nutation of the rotor about the first axis results in rotation of the transmission shaft and the bit box. A related method is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to positive displacement motors (PDMs) for downhole applications in the drilling of oil and gas wells, and more particularly to the power transmission aspects of such motors.

[0003] 2. Description of the Related Art

[0004] The use of downhole drilling motors, particularly PDMs, that convert hydraulic energy from the drilling fluid pumped through a drill string into mechanical power to rotate a drilling bit at the end of the drill string is well known in the art. The energy conversion is accomplished by the reverse application of the Moineau pump principle. Drilling fluid is pumped into the motor's power section at a pressure that causes a helical rotor having “n” lobes to rotate and nutate within a helical stator having “n+1” lobes. The rotor movement is driven by the passage and the compression of the drilling fluid through the cavities defined by the spaces between the rotor lobes and the stator lobes.

[0005] The basic components of a typical PDM power section are illustrated in FIGS. 1-3. Thus, a three-lobe rotor 11 is shown with an outer helical profile 4. The center axis 2 of rotor 11 rotates about center axis 1 within inner helical profile 3 of four-lobe stator 10. Rotor axis 2 defines a circle ◯, the radius of which is equal to the eccentricity e, as the rotor rotates about stator axis 1. The eccentricity is dependent on the diameter of major circle 7 and the diameter of minor circle 5 relative to stator 10, as well as the diameter of major circle 8 and diameter of minor circle 6 relative to rotor 11. While rotor 11 is nutating about stator axis 1, the rotor is also rotating about its own axis 2 as it navigates the stator's inner helical profile 3.

[0006]FIG. 4 illustrates typical PDM power and transmission sections in elevation. Rotor 11 is shown assembled inside stator 10. The internal surface of the stator is generally made of (or lined with) an elastomeric material such as rubber, but other materials such as ceramics and metals are also used in various applications. The stator is mounted in a housing 9. The lower end of rotor 11 is connected to transmission shaft 13 via transmission coupling 12. The axis of the transmission coupling is collinear with the axis 2 of the rotor. The lower end of the transmission shaft is connected to drive shaft 15 via second coupling 14. The second coupling is aligned with drive shaft 15.Drive shaft 15 is supported within lower housing 18 by thrust and radial bearings 16. The rotational force developed in rotor 11 is thus transmitted via transmission coupling 12, transmission shaft 13, second coupling 14, and drive shaft 15 to bit box 17 which directly drives the drill bit (not shown).

[0007] As mentioned above, the second rotational movement of rotor 11 (about its own axis) is utilized to transmit rotation and torque to the bit. Problems reside in this mode of transmission, however, due to the eccentricity inherent in the first rotational movement of the rotor (about axis 1 of the stator). The eccentricity results in a misalignment between the rotor axis 2 and the center axis of the motor, which is collinear with axis 1. This misalignment must be accommodated in transmission shaft 13 and the result is a misalignment angle a shown at 19 in FIG. 4. This misalignment degrades the reliability and solidity of the transmission coupling between the rotor and the transmission shaft, as well as between the transmission shaft and second coupling 14.

[0008] To address this shortcoming, it is a principal object of the present invention to provide a transmission assembly for a PDM that utilizes the rotation of the rotor about the stator axis to produce rotational power.

[0009] It is a further object of the present invention to take advantage of the fact that the rotor speed of rotation about the stator axis is greater than the rotor speed of rotation about its own axis, thereby producing higher speeds to transmit from the PDM power section to the drill bit.

SUMMARY OF THE INVENTION

[0010] The objects described above, as well as various other objects and advantages, are achieved by a downhole motor for converting hydraulic energy from drilling fluid passing through the motor into mechanical power that is useful to rotate a drillbit. The downhole motor includes a tubular housing and a stator mounted within the tubular housing. A rotor is positioned within the stator for rotation relative thereto. The stator has a central first axis and the rotor has a central second axis. The rotor is rotated about the second axis and nutated about the first axis within the stator by drilling fluid passing through the power section. The rotor further has a concentric lower end portion, or pin, thereon. A cylindrical coupling is disposed within the tubular housing beneath the rotor and has an axis substantially aligned with the first axis and an inner cylindrical opening the axis of which is substantially aligned with the second axis, the opening being adapted for receiving the lower end portion of the rotor. A first bearing assembly is disposed in the opening of the coupling for rotatably receiving and supporting the lower end portion of the rotor, whereby nutation of the lower end portion of the rotor about the first axis induces rotation of the coupling about the first axis. A second bearing assembly is disposed between the coupling and the tubular housing for supporting rotation of the coupling relative to the tubular housing. A transmission shaft is connected to the coupling for transmission of the rotation to the bit box. In this manner, nutation of the rotor about the first axis results in rotation of the transmission shaft and the bit box.

[0011] In a preferred embodiment, the cylindrical coupling has a peripheral opening therein for passage of drilling fluid through the cylindrical coupling. The peripheral opening is preferably shaped to optimize the structural integrity of the cylindrical coupling while minimizing the resistance to flow of the drilling fluid therethrough.

[0012] In another aspect, the present invention contemplates a method for converting hydraulic energy from drilling fluid passing through a drill string into mechanical power that is useful to rotate a drillbit. The method includes the steps of placing a tubular housing in the drill string, and mounting a stator within the tubular housing. A rotor is positioned within the stator for rotation relative thereto. The stator has a central first axis and the rotor has a central second axis. The rotor is rotated about the second axis and nutated about the first axis within the stator by drilling fluid passing through the stator. The nutation of the rotor about the first axis is converted into rotation of a transmission assembly and bit box.

[0013] In a preferred embodiment, the converting step includes equipping the rotor with a concentric lower end portion, or pin, whereby nutation of the rotor about the first axis induces nutation of the lower end portion about the first axis. A coupling is placed within the tubular housing beneath the rotor. The coupling has an axis substantially aligned with the first axis and an inner opening the axis of which is substantially aligned with the second axis. The opening is adapted for receiving the lower end portion of the rotor, whereby nutation of the lower end portion of the rotor induces rotation of the coupling about the first axis. The rotation of the coupling is applied to a transmission assembly to rotate the bit box.

[0014] The method preferably further comprises the steps of placing a first bearing assembly in the opening of the coupling for rotatably receiving and supporting the lower end portion of the rotor, and placing a second bearing assembly between the coupling and the tubular housing for supporting rotation of said coupling relative to said tubular housing.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0015] The manner in which the present invention attains the above recited features, advantages, and objects can be understood with greater clarity by reference to the preferred embodiment(s) thereof which are illustrated in the accompanying drawings.

[0016] It is to be noted however, that the appended drawings illustrate only typical embodiment(s) of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0017] In the drawings:

[0018]FIG. 1 is a sectional plan view of a PDM rotor;

[0019]FIG. 2 is a sectional plan view of a PDM stator;

[0020]FIG. 3 is a sectional plan view of the rotor of FIG. 1 assembled within the stator of FIG. 2;

[0021]FIG. 4 is a sectional elevational view, partly in section, of the rotor-stator assembly of FIG. 3 connected to a bit box via a transmission assembly in a conventional manner;

[0022]FIG. 5 is a sectional elevational view, partly in section, of a rotor-stator assembly connected to a bit box via a transmission assembly in accordance with the present invention; and

[0023]FIG. 6 is a sectional plan view taken along line 6-6 in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Referring now to the FIG. 5, tubular stator housing 9 is shown having helically profiled stator 10 mounted therein. The stator has a central first axis 1. Helically profiled rotor 11 is positioned within stator 10 for rotation relative thereto. The rotor has a central second axis 2 and is rotated about second axis 2 and nutated about first axis 1 within stator 10 by drilling fluid passing through the stator. Rotor 11 further has a concentric lower end portion or pin 23 thereon that is also rotated about second axis 2 and nutated about first axis 1.

[0025] Cylindrical coupling 22 is disposed within tubular stator housing 9 beneath rotor 11 and has a central axis substantially aligned with first axis 1 and includes inner cylindrical opening 31 the axis of which is substantially aligned with second axis 2. Opening 31 is adapted for receiving lower end portion or pin 23 of rotor 11. By this positioning, pin 23 transmits the rotor's nutating movement to coupling 22.

[0026] First bearing assembly 20 includes thrust and radial bearings and is disposed in opening 31 of coupling 22 for rotatably receiving and supporting pin 23 of rotor 11. The first bearing assembly take the rotor hydraulic thrust load, freely supports rotor 11 during its rotational movement about second axis 2, and transmits the nutating movement of pin 23 to the coupling 22, inducing rotation of coupling 22 about the first axis. Rotation of coupling 22 is supported inside tubular housing 9 by second bearing assembly 21, including thrust and radial bearings, permitting the coupling to rotate relative to the tubular housing.

[0027] The bottom end of the coupling is aligned with tubular housing 9 and first axis 1, and is connected to bit box 17 through transmission shaft 13. 1. In this manner, nutation of rotor 11 about first axis 1 results in rotation of the transmission shaft and the bit box.

[0028]FIG. 6 shows a sectional view of coupling 22. Thus, rotor pin 23 and first bearing assembly 20 (neither of which is shown in FIG. 6) are disposed inside central bore 31, which is aligned with second axis 2. Second bearing assembly 21 (not shown in FIG. 6) supports coupling 22 on the circular outer surface 32 of the coupling, which is aligned with first axis 1. Peripheral slot or opening 30 is formed in the body of coupling 22 to allow for the passage of drilling fluid. The shape of this peripheral opening is designed to minimize erosion and frictional losses due to the drilling fluid interaction and to maximize the structural integrity of coupling 22.

[0029] The present invention therefore utilizes the rotation of second (rotor) axis 2 about first (stator) axis 1 to transmit power to bit box 17. Since that rotation is defined relatively to the first axis, which is the same as the tubular housing axis, no flexible coupling is needed to transmit the movement and the torque to the bit, unless a bend is needed in the motor assembly. If no bend is needed, the rotor may thus be directly linked to transmission shaft 13 and bit box 17, resulting in the removal of the longer, conventional transmission section and in the shortening of the motor assembly as a whole.

[0030] Those skilled in the art will further appreciate that the combination rotation/nutation movement about two axes 1, 2 is dictated by the respective numbers of lobes on rotor 11 and stator 10. For the 3:4 lobe PDM shown in FIG. 1-3, the nutational speed of rotor 11 about stator axis 1 will be three times faster than the rotational speed of the rotor about its own axis 2. The main consequence is that the free rotational speed of the transmission shaft and bit box, as compared to a standard design at the same flow rate, are increased by an amount which is equal to the number of lobes on the rotor. This increased speed makes the present invention suitable for applications with high speed PDC or diamond-impregnated drill bits.

[0031] Since the bit rotational speed may be increased by a substantial amount, the invention makes possible the choice of a downhole motor having a very high number of lobes for normal to high-speed applications. This in turn greatly improves the reliability of the power section since a very high number of lobes stator improves the heat dissipation by reducing the variation in the thickness of the rubber.

[0032] In view of the foregoing it is evident that the present invention is well adapted to attain all of the objects and features hereinabove set forth, together with other objects and features which are inherent in the apparatus disclosed herein.

[0033] As will be readily apparent to those skilled in the art, the present invention may easily be produced in other specific forms without departing from its spirit or essential characteristics. The present embodiment is, therefore, to be considered as merely illustrative and not restrictive. The scope of the invention is indicated by the claims that follow rather than the foregoing description, and all changes which come within the meaning and range of equivalence of the claims are therefore intended to be embraced therein.

[0034] For example, with reference again to FIG. 5, second bearing assembly 21 may also be used as the main motor bearing assembly since a conventional transmission assembly (see FIG. 4) is obviated by the present invention and the motor assembly as a whole becomes shorter. A longer, conventional transmission assembly may be used if the motor has to be bent for well directional control, with the advantage of extending the life of the transmission since there is no more misalignment (see FIG. 4 again) due to the rotor eccentric movement. 

What is claimed is:
 1. A downhole motor for converting hydraulic energy from drilling fluid passing through the motor into mechanical power that is useful to rotate a drillbit, comprising: a tubular housing; a stator mounted within said tubular housing, said stator having a first axis; a rotor positioned within said stator for rotation relative thereto, said rotor having a second axis and being rotated about the second axis and nutated about the first axis within said stator by drilling fluid passing through said stator, said rotor further having a concentric lower end portion; a cylindrical coupling disposed within said tubular housing beneath the rotor and having an axis substantially aligned with the first axis and an inner cylindrical opening the axis of which is substantially aligned with the second axis, the opening adapted for receiving the lower end portion of said rotor; a first bearing assembly disposed in the opening of said coupling for rotatably receiving and supporting the lower end portion of said rotor, whereby nutation of the lower end portion of said rotor about the first axis induces rotation of said coupling about the first axis; a second bearing assembly disposed between said coupling and said tubular housing for supporting rotation of said coupling relative to said tubular housing; and a transmission shaft and bit box connected to said coupling for rotation, whereby nutation of the rotor about the first axis results in rotation of said transmission shaft and said bit box.
 2. The downhole motor of claim 1, wherein said cylindrical coupling has a peripheral opening therein for passage of drilling fluid through said cylindrical coupling.
 3. The downhole motor of claim 2, wherein the peripheral opening is shaped to optimize the structural integrity of said cylindrical coupling while minimizing the resistance to flow of the drilling fluid therethrough.
 4. In a downhole motor including a tubular stator housing, a helical stator having a first axis and connected to the stator housing, and a helical rotor having a second axis, the rotor being rotated about the second axis and nutated about the first axis within the stator by drilling fluid passing through the stator, an apparatus for transmitting torque developed at the rotor to a bit box, comprising: a pin connected to the lower end of the rotor, said pin having an axis aligned with the second axis whereby nutation of the rotor about the first axis induces nutation of said pin about the first axis; a cylindrical coupling disposed within the stator housing beneath the rotor and having an axis substantially aligned with the first axis and an inner cylindrical opening the axis of which is substantially aligned with the second axis, the opening adapted for receiving said pin; a first bearing assembly disposed in the opening of said coupling for rotatably receiving and supporting said pin, whereby nutation of said pin about the first axis induces rotation of said coupling about the first axis; a second bearing assembly disposed between said coupling and the stator housing for supporting rotation of said coupling relative to the stator housing; and a transmission shaft and bit box connected to said coupling for rotation, whereby nutation of the rotor about the first axis results in rotation of said transmission shaft and said bit box.
 5. The downhole motor of claim 1, wherein said cylindrical coupling has a peripheral opening therein for passage of drilling fluid through said cylindrical coupling.
 6. The downhole motor of claim 2, wherein the peripheral opening is shaped to optimize the structural integrity of said cylindrical coupling while minimizing the resistance to flow of the drilling fluid therethrough.
 7. A method for converting hydraulic energy from drilling fluid passing through a drill string into mechanical power that is useful to rotate a drillbit, comprising: placing a tubular housing in the drill string; mounting a stator within the tubular housing, the stator having a first axis; positioning a rotor within the stator for rotation relative thereto, the rotor having a second axis and being rotated about the second axis and nutated about the first axis within the stator by drilling fluid passing through the stator; and converting the nutation of the rotor about the first axis into rotation of a transmission assembly and bit box.
 8. The method of claim 7, wherein the converting step includes: equipping the rotor with a concentric lower end portion, whereby nutation of the rotor about the first axis induces nutation of the lower end portion about the first axis; placing a coupling within the tubular housing beneath the rotor, the coupling having an axis substantially aligned with the first axis and an inner opening the axis of which is substantially aligned with the second axis, the opening adapted for receiving the lower end portion of the rotor, whereby nutation of the lower end portion of the rotor induces rotation of the coupling about the first axis; and applying the rotation of the coupling to a transmission assembly and bit box to rotate the transmission assembly and bit box.
 9. The method of claim 8, further comprising the steps of: placing a first bearing assembly in the opening of the coupling for rotatably receiving and supporting the lower end portion of the rotor; placing a second bearing assembly between the coupling and the tubular housing for supporting rotation of said coupling relative to said tubular housing. 